The invention refers to a variable displacement screw-type compressor.
In installations using a compressed medium as the pressure gas, it is often necessary to regulate the pressure prevailing in the installation. This is either done by bleeding a certain amount of pressure from the installation or returning it via the bypass, or by changing the feed rate of the pressure gas.
If the compressor used in such an installation is a screw-type compressor, the feed rate may be adjusted by changing the number of rotations of the screw-type compressor. However, this adjustment has its limit where too small numbers of rotation of the screw-type compressor cause the efficiency of the screw-type compressor to fall below an acceptable value.
Since the bleeding and the bypassing of the pressure gas cause unacceptable energy consumption, anyway, and, on the other hand, a broad adjustment range is imperative, screw-type compressors have been developed, in which the feed rate provided by the screw-type compressor can be adjusted by the compression ratio of the screw-type compressor.
DE-05 25 26 175 shows a screw-type compressor with a variable volumetric capacity, wherein two meshing rotors are arranged in a housing. In this screw-type compressor, a return flow channel extends in parallel to the rotor axes of the screw-type compressor, which channel may be communicated with the interior of the housing through closable openings. In the open state, the openings spaced in the axial direction of the rotors allow for a return flow of the medium to be compressed through the return flow channel to the intake side of the screw-type compressor. Thereby, the compression of the medium flown in between the screws and the inner wall of the housing starts sooner or later, depending on the opening state of the openings so that the feed rate provided by this screw-type compressor is variable. However, the openings, the ends of which at the overflow channel side may be closed by means of a control piston, are also open towards the interior of the housing when in their closed state, whereby they form overflow pockets that cause a return flow of the medium to be compressed even at a delivery volume of 100%. Therefore, the screw-type compressor has a poor efficiency.
DE-PS 35 16 636 on which the precharacterizing part of claim 1 is based, discloses another variable displacement screw-type compressor. In the same, a primary and a secondary rotor are provided in a housing. In this screw-type compressor, the medium to be compressed is conveyed from an inlet channel to an outlet channel, the inlet channel having two housing segments displaceable along the rotor axes. The housing segments each extend in the housing over the entire rotor length and are floatingly supported at one end. The pressures occurring transversely to the longitudinal extension of the housing segments and being received through guiding paths, however, cause bending loads on the housing segments. In operation, displacing the housing segments towards the outlet channel causes a channel wall defining the inlet channel to be displaced such that the inlet channel is prolonged and the compression starts later. Due to the later start of the compression, the compression ratio, and thereby the feed rate of the screw-type compressor, is modified.
To make even small changes in the displaced volume, it is necessary to displace the respective entire housing segment extending over the full rotor length. The unilaterally floating support of the housing segments necessary therefor, however, has the disadvantage of a high production effort. Further, the control segments that extend through the pressure side of the screw-type compressor are induced to vibrate by the high pulsating pressures prevailing at the pressure side of the screw-type compressor, the vibrations causing the guiding paths to become unsteady. Finally, it is another disadvantage that the housing segments cause a great structural length of the screw-type compressor and impose a limit to the freedom of structural design on the pressure side of the screw-type compressor.